Because Magnus is a girl-to-boy trans and his girlfriend is a boy-to-girl trans. It's straight because Magnus is to be seen as a boy and his girlfriend should be treated like a girl. But even if they identified with their biological genders, it'd still be opposite genders, ergo still straight no matter what way you look at it. :I

It's not 'looks', it more of an 'identifies as'. Some trans people do manage to look their opposite gender successfully, though, but no always. Anyway For these two it can be either way. Magnus might not have the equipment of a dude but can still be a dude and vice versa for his girlfriend.

Basically, Magnus was born female. He has breasts, vagina, everything that goes along with it. He identifies as male though - it's how he acts, dresses, sees himself and how he should be treated. He binds his chest and quite possible (though not necessarily, I don't really know) takes hormones.

Meanwhile, his girlfriend was born male. She has no natural breasts but does have a penis, etc. She, however, identifies as female, and is how she acts, dresses and sees herself. So while she may have "male parts" she should be treated as female.